In the construction of pipelines it is necessary to perform welding on the pipeline joints both internally and externally. Access is not a problem in performing the external weld, but it is a very substantial problem in performing the internal weld pass. Since human access is impossible in most applications, there have been developed automated internal welding machines which perform this operation. Such an internal welding machine is shown in U.S. Pat. No. 3,612,808 to Nelson. An internal automatic welder is described in U.S. Pat. No. 4,525,616 to Slavens.
Although internal welders are in wide use throughout the world in the construction of pipelines, these welders have drawbacks which limit their productivity and increase the cost of operation. A specific limitation in the use of automated welding equipment, which is particularly acute for internal welders, is the speed at which the welding pass is performed. In an internal welding operation it is difficult and expensive to make repairs for defective welds. It is, therefore, imperative that the equipment work properly for a very high percentage of the welding operations. This, however, has led to a productivity trade-off. At slower travel speeds for the automatic welder, there is a higher probability that the arc will be struck and properly initiated. If higher traveling speeds are selected, the probability of properly striking an arc is reduced. Therefore, since reliability is of utmost importance, the travel speed of automatic internal welders has been set to a relatively slow rate to ensure proper arc initiation. However, the slow rate required for high reliability arc initiation causes the entire weld sequence to be excessively time consuming. Thus, a principal drawback of conventional automated welding equipment, and in particular internal automated welders, is the slow travel speed necessary for high quality welds.
A further limitation in the use of internal welders is the complex and expensive electronic control system required to operate the welders. A substantial number of mechanical and electrical operations must be carried out in using an internal welder machine. These steps include moving the machine along the pipeline, properly aligning the machine to the end of the pipe joint, clamping the machine to one pipe joint and then to a next pipe joint, positioning the internal welders to the appropriate positions for starting and stopping the weld passes, clamping the machine within the pipes, initiating welder operation including starting travel of the welders, providing feed wire and providing a shielding gas. Further, the arc must be constantly monitored for each of the welders and provision must be made for starting and stopping the welders at appropriate locations. Almost all of these functions must be initiated, monitored and stopped through electronic equipment. However, an internal welder must function in a severe environment. It must be used in adverse weather conditions with extremes of temperature, humidity and exposure to dust and smoke. The unit is also subject to extreme physical stresses and rough handling. The electronic control system in addition is subjected to a harsh electrical environment due to the static, transients and high currents produced by arc welding. The extremely high current levels used in such welding create substantial magnetic fields that can affect the operation of electronic components. In addition, welding equipment of this nature is often used in remote locations and it is difficult to provide maintenance and spare parts. Thus, simplicity and a minimum number of parts for the control system is of great importance.
Further, the large number of control operations required to operate an internal welder result in the creation of a very large cable bundle having numerous wires for operating the large number of solenoids, switches and other electronic components. A large cable bundle of this type can be accommodated in large internal welders, but on smaller units, such as 20 inch and smaller internal welder units, such a cable bundle is very difficult to accommodate. It can interfere with the operation of the internal welder unit and is more subject to damage in operation. Thus, there is a distinct need for an improved electronic control system for automated welding operations and in particular, there is a need for a more reliable, less complex and physically smaller control system for an internal welding machine.